Aromatase converts androgen to estrogen. Aromatase is expressed at a higher level in breast cancer tissue than in benign tissue. In situ estrogen biosynthesis in tumor tissue has been shown to play both an autocrine and an endocrine role in promoting tumor growth. Suppression of in situ estrogen biosynthesis can be achieved by the prevention of aromatase expression in breast tumors or by the inhibition of aromatase activity. The regulation of aromatase expression is different in tumor tissue and benign tissue. Based on results generated from this and other laboratories, it is hypothesized that in normal breast stromal cells, aromatase expression is driven by a promoter (1.4) that is regulated by glucocorticoid, and the action of promoters 1.3 and II is suppressed by a silencer negative regulatory element. However, in cancer tissue, cAMP production increases and aromatase promoters are switched to cAMP-dependent promoters, i.e., 1.3 and II. In Aim 1, the applicant proposes to perform a thorough study to determine the regulatory mechanism of promoters 1.3 and II In breast cancer cells, based on important information obtained during the previous grant period. It is hypothesized that understanding of the regulatory mechanism of promoters 1.3 and II will lead to the development of breast cancer treatment strategies by selectively suppressing aromatase/estrogen formation in breast cancer cells. During the last five years, aromatase inhibitors have been demonstrated to be superior to tamoxifen with the treatment of hormonal dependent breast cancer. Furthermore, steroidal inhibitors and nonsteroidal inhibitors have been shown to maintain their efficacy when used sequentially. In Aim 2, the applicant proposes to perform x-ray structure analysis, computer modeling and site-directed mutagenesis experiments to determine how different inhibitors interact with aromatase. It is hypothesized that results generated from structure-function studies will help us to better understand how different inhibitors interact with the enzyme and provide critical structural information for the design of the next generation of aromatase inhibitors for breast cancer treatment. In addition, while these new generations of aromatase inhibitors are shown to be useful in the treatment of hormonal responsive breast cancer, resistance to such endocrine therapy still develop. In Aim 3, the applicant proposes to perform a careful and thorough analysis of the results obtained from our microarray analysis, to identify and functionally confirm the roles of genes Involved In resistance. It is hypothesized that these studies will produce valuable molecular information regarding the mechanisms of aromatase inhibitor resistance, and the information will help design approaches to reduce resistance and improve the efficacy of aromatase inhibitor treatments of breast cancer.